This invention is a process for increasing the heat deflection temperature of plastic cups made from amorphous polyethylene terephthalate. Such cups are generally thermoformed from sheet material, under conditions which result in the polyethylene terephthalate existing in a largely amorphous state, the degree of crystallinity being quite low. As a consequence such cups have a low heat deflection temperature of about 155.degree. F.
In using cups made of polyethylene terephthalate for terminal sterilization of foods it may be necessary to expose the cups to temperatures up to 250.degree. F. for low acid foods and up to 212.degree. F. for high acid foods. By contrast, in using cups made from polyethylene terephthalate for aseptic packaging of food products it is necessary to sterilize the cups prior to filling them with food. Sterilization for aseptic packaging may generally be carried out by subjecting the cups to contact with a hot liquid sterilizing agent. However, the temperature at which the sterilization is accomplished must be below the heat deflection temperature of the polyethylene terephthalate cup. Thus, using cups with a low heat deflection temperature requires a low sterilization temperature, and for the sterilization to be effective, extended time periods are required. Because of the logarithmic order of death of microorganisms as a function of temperature, if the heat deflection temperature of the cups could be raised to 170.degree. F., sterilization time would be materially shortened, and the aseptic packaging operation speeded up significantly.